Capacitive approach and/or contact sensors measure, by generating and measuring electric alternating fields, the approach of an object to the sensor electrodes or the touch of the sensor electrodes by an object. Capacitive sensor devices known from prior art, especially when they are employed in mobile electric devices, have the disadvantage that the quality and the stability of the detection of an approach or a contact depends much on the ground reference of the mobile device or on the connection between the ground of the device and an operator of the mobile device.
For example a connection of peripherals at a mobile device may entail that the device obtains a ground reference, which can negatively affect the measurement or detection of an approach and/or contact. A connection between the ground of the devices and an operator of the device can take place for example by a ground coupling, for instance by an accumulator, of the electric device onto the human body.
In this respect it is particularly disadvantageous that the ground reference of the mobile device or the ground coupling on the human body can vary during the operation of the electric device, for example if during the operation a peripheral having a ground reference is connected to the electric device. This may entail that a detection of an approach or a contact which at first is correct leads to a wrong detection. For example the formation of a ground reference of the mobile device during the operation may entail that for instance a grasp of the mobile device by a hand is not recognized as such by mistake.
In capacitive sensor devices known from prior art one tries with two electrodes (transmitting electrode and reception electrode) to solve this problem by the fact that the sensor device substantially can be operated in two different modes. If it is known that the device is grounded, the capacitive sensor device can be operated in a so-called absorption mode, in which a signal reduction of an electric signal at the reception electrodes is evaluated. The signal reduction substantially arises by the deduction of the alternating electric field between the transmitting electrode and the reception electrode by means of a body approaching the electrodes, which presents a ground reference. On the other hand if it is known that the device is not grounded, the capacitive sensor device can be operated in a so-called transmission mode. In the transmission mode an approach of an electrically conductive object, which may be grounded, entails a rise of an electric signal tapped at the reception electrodes.
This method has however the disadvantage that mostly it is not known if the electric device has a ground reference or how strong a ground coupling of the electric device is. In order to operate a capacitive sensor device in the correct operating mode, in the methods known from prior art it is determined at first if a ground reference of the electric device is present or not. In order to detect the ground reference or the strength of the ground coupling, however additional measures must be taken, which entail a considerable rise of the cost of the capacitive sensor device.